1. Field of the Invention
This invention relates to a pitch fiber bundle which is one kind of the precursor of carbon fibers, a pitch type carbon fiber bundle, and a method for the production thereof. Particularly, this invention relates to a pitch fiber bundle which is one kind of the precursor of carbon fibers of a small size fit for the production of woven fabrics of carbon fibers and a carbon fiber prepreg of a low areal weight ("Metsuke" in Japanese), a pitch type carbon fiber bundle, and a method for the production of carbon fibers of a small size, i.e. a method for producing, at a low cost with high operational efficiency, a pitch fiber bundle which is one kind of the precursor of carbon fibers of a small size, a pitch type carbon fiber bundle, and carbon fibers of a small size. The pitch type carbon fibers which are obtained by this invention are fit for the production of lightweight woven fabrics of carbon fibers and carbon fiber prepreg and are used advantageously in various industrial fields covering sports, leisure, and aerospace technology.
2. Description of the Related Art
The pitch type carbon fibers, like pitch fibers which are the precursor thereof, infusibilized fibers, and carbonized fibers, are weak and very difficult to handle as compared with the PAN type carbon fibers which have acrylic fibers as the precursor.
Heretofore, the production of pitch type carbon fibers of a small size has required to decrease the number of filaments during the process of spinning (or extrusion), with the result that the productivity will be conspicuously lowered and, at the same time, the necessity of handling feeble fibers of a small size will be incurred. The production of carbon fibers of a small size on a commercial scale, therefore, embraces factors for degrading productivity and boosting cost. It, therefore, has entailed the problem that lightweight woven fabrics of carbon fibers and carbon fiber prepreg will become highly expensive.
As a measure to cope with this problem, a method which obtains carbon fibers of a small size by producing a carbon fiber bundle provisionally and then dividing this bundle into several bundles has been proposed. The pitch type carbon fibers generally stretch only meagerly, however, because they possess a higher modulus of elasticity than the PAN type carbon fibers. The carbon fiber bundle, therefore, is incapable of being stably divided into smaller bundles of a prescribed size because it copiously produces broken fibers during the course of division.
JP-A-01-250,417 proposes a method which comprises paralleling and twisting not less than two fiber bundles as the precursor of the PAN type carbon fibers into a cord, doubling such cords and firing the doubled cords, and thereafter dividing the fired doubled cords into the individual carbon fiber bundle cords. The carbon fibers produced by this method, however, have the problem that they will not fit fabrication into lightweight prepregs, though they are suitable as sewing threads. Similarly JP-A-09-273,032 and EP-A-0835953 propose a fiber bundle for use in the PAN type carbon fibers, which retains the shape of one tow and permits division into a plurality of small tows prior to use. This fiber bundle, however, encounters great difficulty in being adapted for pitch type carbon fibers because it is required to allow impartation of crimps never attainable with pitch fibers and endure the impact of a winder to be used for doubling small tows.
Then, JP-A-01-229,820 proposes pitch type carbon fibers which have the number of filaments of less than 1,000. The method used for producing these fibers, however, has the problem that the component fibers are fated to succumb to agglutination. It has another problem that the productivity is prominently impaired because extremely brittle pitch fibers are doubled after the spinning which can be carried out at a relatively high speed.